The Patent Specification DE 41 23 208 C 2 describes a compressor system in which at least one exhaust turbocharger is located on the drive side in the suction and/or pressure piping of a compressor on the exhaust side of an internal combustion engine and supplies the compressor with precompressed air and/or subsequently compresses the air compressed by the compressor.
The drawback here is that the exhaust backpressure of an internal combustion engine is increased when one or more exhaust turbochargers are placed in its exhaust train and so its mechanical performance is reduced. The exhaust backpressure rises to about 1.5 bar and produces a reduction in mechanical performance due to the increased working of the internal combustion engine pistons, this reduction being about 20% in traditional internal combustion engines with a low mean effective pressure. In addition, the exhaust backpressure leads to poorer filling of the internal combustion engine with fresh air, which means that the fuel quantity has to be reduced for achievement of complete combustion of fuel and the performance of the internal combustion engine drops. In traditional, naturally aspirated engines, the loss of performance caused in this way amounts to about 10%.
Thus a total loss of performance of about 30% results in the internal combustion engine. This loss of performance in the internal combustion engine and the resulting compression loss of the compressor driven by the internal combustion engine is too high, since it cannot be compensated by additional compression gained from use of exhaust turbochargers.
The higher construction costs for the compressor system under Patent Specification DE 41 23 208 C 2 in comparison with a traditional compressor system without an exhaust turbocharger upstream of the compressor mean that such a system is very uneconomical and inefficient.
U.S. Pat. No. 2,849,173 describes a compressor system in which the exhaust gases of a thermal engine driving a piston compressor also drive a gas turbine which supplies the piston compressor with precompressed air via a compressor driven by the gas turbine. A multistage axial-flow compressor is shown here as the upstream compressor. With the technical method shown, the drawback is that such a compressor system constitutes an expensive and unwieldy solution because of the multistage axial-flow compressor and is thus unsuitable for use as a mobile compressor system. In addition, the internal combustion engine has a large loss in performance due to the high exhaust backpressure caused by the downstream gas turbine compressor stage. So this technical method does not achieve any improvement in the efficiency of a compressor system by upstream connection of a precompression stage driven by exhaust gases.
U.S. Pat. No. 3,204,859 also describes an arrangement in which an exhaust turbocharger is driven by the exhaust gases of a thermal engine. This invention shows the division of air precompressed by a turbocharger between the engine on the one hand and the compressor itself on the other. On the one hand, fresh air is taken in by the compressor and on the other it is supplied with precompressed air by the turbo-compressor if the boost pressure exceeds that required by the engine, i.e. there is surplus precompression pressure which leads to the opening of a valve to the compressor. The drawback in this arrangement is that only part of the precompressed air is available for the compressor itselfxe2x80x94and only when boost pressure has reached a certain level. It is also a disadvantage that the precompressed air in the compressor is fed into an intermediate area. This produces problems with the sealing and dimensioning of the machine, since there is a rise in the volumetric flow in the intermediate area.
U.S. Pat. No. 4,563,132 describes an engine-compressor unit in which two turbochargers driven by the exhaust gases of the engine are located in the compressor intake pipe. As in the patent specification U.S. Pat. No. 3,204,859, there is a division of precompressed air between engine and compressor with the help of a valve here too. The drawback here is that the engine and compressor run at the same speed, so that no variation of pressure and quantity supplied is possible. Thus the engine-compressor unit is not suitable for use as a mobile compressor unit but designed for engine operation, since the compressor side is not ideal.
This invention relates to a compressor system for production of compressed air, consisting of a compressor driven by an internal combustion engine, connected by piping to compressed air discharge points and supplied with precompressed air by an exhaust turbocharger on the drive side connected on the exhaust side of the internal combustion engine.
Such a compressor system is designed to use process heat to increase the efficiency of the system and thus save energy.
The aim of the invention is to achieve an improvement in the overall efficiency of a compressor system with an exhaust turbocharger upstream of the compressor, thus enabling cost-efficient application of such a compressor system.
The invention achieves this through the characteristic features under Patent claim 1. The sub-claims 4 to 9 describe advantageous forms of the invention.
Thus overall efficiency is improved by arranging two exhaust turbochargers in line so that the exhaust gas mass flow of a diesel engine drives the first exhaust turbocharger first, which supplies compressed air to this diesel engine, and then drives the second exhaust turbocharger which supplies the compressor with precompressed air. In this arrangement, the turbine-side wheel geometry, such as the wheel diameter, of the exhaust turbocharger upstream of the displacement compressor is matched to the different mass flows on the turbine side and compressor side of the exhaust turbocharger, and the turbine-side wheel geometry, such as the outside wheel diameter, of the exhaust turbocharger upstream of the diesel engine is matched to the exhaust backpressure and temperature resulting from the downstream exhaust turbocharger of the displacement compressor.
Thanks to the boost in the mean effective pressure of the diesel engine achieved in this way, the loss of mechanical performance is reduced to about 15%. Loss of performance due to poorer filling of the diesel engine with fresh air can be avoided by higher boost pressure. Thus the total loss of performance of the diesel engine resulting from exhaust back pressure is reduced to about 15%.
An additional reduction in diesel engine losses can be achieved by intermediate cooling of the charge air before it enters the diesel engine. The mean effective pressure of the diesel engine is boosted in this way, so that the exhaust backpressure of about 1.5 bar only leads to a loss in mechanical performance of about 10%. Neither do any losses result from poor filling of the diesel engine with fresh air in this case, so that total loss in performance is reduced to about 10%.
The compressor performance gained in the exhaust turbocharger before the displacement compressor corresponds to about 25% of the mechanical performance of the engine. As a result, a gain in performance of 15% is produced for the compressor system forming the subject of the invention.
In addition, the compressor system forming the subject of the invention, with a diesel engine and an exhaust turbocharger, has the advantage that the useful speed range of the diesel engine is greater here. A change in the speed of the diesel engine is of great importance, since the intake of compressed air from the compressor system fluctuates and so the engine speed is increased or reduced in accordance with the intake of compressed air. In traditional compressor systems, the engine speed can only be reduced by 30% to 40%, related to a maximum useful speed, since the compressor requires an approximately equal torque for provision of constant end pressure extending over the whole speed range of the engine. For example, the useful speed range is between 2500 revolutions per minute and 1500 revolutions per minute.
In the compressor system with an additional exhaust turbocharger, the torque requirement consists of two elements, namely that of the worm compressor and that of the additional turbocharger. With decreasing speed of the diesel engine, exhaust turbocharger performance sinks and thus the exhaust backpressure too. The falling exhaust backpressure then produces a rise in the mechanically useful torque of the diesel engine, so that, even in this operating condition, there is adequate engine torque for provision of the desired end pressure by the compressor. Thus it is possible to extend the speed range of the diesel engine to include low speeds, so that a speed range between 2500 revolutions per minute and 1000 revolutions per minute can be used. This corresponds to an increase in the useful speed range by about 50%. Precisely because there is only a sporadic intake of compressed air from the compressor system, the using of lower speeds of the diesel engine leads to a fuel-saving potential of up to 30% in comparison with traditional compressor systems.
Another advantage of the compressor system forming the subject of the invention is the use of a displacement compressor. In compressor systems driven by internal combustion engines, it is customary to regulate the supplied amount of compressed air via machine speed too. However, a reduction in engine speed and a resulting loss of intake pressure also produce a reduction in the compressor performance of the exhaust turbocharger located upstream of the compressor. The resulting lower intake pressure leads to generation of a higher pressure ratio by the compressor to achieve the desired constant end pressure.
The compressor system forming the subject of the invention solves this problem by using a displacement compressor, which is able to supply constant end pressure even when there are changes in pressure conditions upstream of the compressor, since there is practically no exhaust chamber. In particular, use of a liquid-injected displacement compressor is recommended, such as a worm or rotary compressor.
Another advantage of use of a liquid-injected displacement compressor is that even a one-stage construction version can provide the required compression, meaning that the whole compressor system can be kept small in size.
The overall efficiency of such a machine is improved by the positioning in line, on the exhaust gas side, of a first exhaust turbocharger (20), supplying precompressed air to a diesel engine (40), and a second exhaust turbocharger (21), supplying precompressed air to a liquid-injected displacement compressor (50), with the turbine-side wheel geometry, such as the outside wheel diameter, of the exhaust turbocharger (20) being matched to the exhaust backpressure and temperature resulting from the downstream exhaust turbocharger (21) of the displacement compressor (50) and the turbine-side wheel geometry, such as the wheel diameter, of the exhaust turbocharger (21) being matched to the different mass flows on the turbine side and compressor side of the exhaust turbocharger (21).